The present invention relates to a crosslinked polyvinyl chloride resin foam having a high expansion ratio and excellent vacuum formability, flexibility, compression recovery, thermal resistance and hot adhesion, particularly high-frequency sealability and free from bleeding out of a plasticizer.
As is well known, polyvinyl chloride foams have been used widely in the production of automobile trims and furniture in virtue of their excellent flame retardancy and mechanical properties. However, the production of polyvinyl chloride resin foams having a high expansion ratio and excellent thermal resistance, flexibility and formability has technical difficulties in crosslinking and plasticizing the polyvinyl chloride foam. More particularly, crosslinking is necessary in the production of a polyvinyl chloride foam having a high expansion ratio with a decomposable blowing agent because the melt viscosity of the polymer must be increased to one suitable for forming a gas. It is believed that a reason why the expansion ratio of non-crosslinked polyvinyl chloride foams is as low as 2 to 3 is that the melt viscosity of the polymer is drastically lowered in the foaming step to make the retention of the original shape of the sheet impossible and a gas generated from the decomposed blowing agent is released from the sheet surface.
A process which comprises kneading polyvinyl chloride with a polyisocyanate, a blowing agent, an unsaturated acid anhydride, a polymerizable monomer and a halogenated saturated hydrocarbon, filling the obtained mixture in a pressure mold, heating and compressing the same with a press to effect gelation of the polymer and decomposition of the blowing agent, and heating the product again with hot water or steam to form a foam has been disclosed in the specifications of Japanese patent publications Nos.39-223370 and 41-12632. However, this process can be carried out only in batches and, therefore, its productivity is low and the cost of the foam is high. Further, when the crosslinking and foaming are effected under atmospheric pressure, the flowing agent is decomposed before the crosslinking reaction proceeds and, as a result, a gas generated thereby cannot be incorporated as foams into the molten polymer but released from the reaction system to make the production of a foam of a high expansion ratio impossible. Thus the conditions of this process are limited, namely, the crosslinking is effected by heating under pressure while the foaming is effected by heating under ambient pressure.
A process for producing crosslinked polyvinyl chloride resin foam by irradiation with electron beams is disclosed in the specifications of Japanese patent publications Nos. 46-18620 and 48-4863 and Japanese patent application Kokai (=laying-open) publication No. 48-1729. However, this process wherein electron beams are employed has defects that hydrochloric acid is formed by the decomposition of the polyvinyl chloride resin, which is competitive with the crosslinking reaction, that the obtained foam is colored with hydrochloric acid thus formed, and that the color is darkened from brown to black as the energy of irradiation is increased. In addition, the practical properties such as vacuum formability, thermal resistance, cold resistance (low temperature characteristic), hot adhesion and compression recovery of the foam crosslinked by irradiation with electron beams are unsatisfactory yet.
When these polyvinyl chloride resins are molded into a continuous sheet and the sheet is crosslinked and foamed under atmospheric pressure to form a foamed sheet, it is necessary to carry out melting and kneading under such conditions that the blowing agent will not be decomposed and then form a continuous sheet by extrusion or calendering. Since, however, the vinyl chloride resin usually contains a large amount of a plasticizer such as dioctyl phthalate, dibutyl phthalate, dioctyl adipate or epoxidized soybean oil, the plasticizer causes bleeding to a large extent and deposits on the sheet surface in the sheet-forming or foaming step. As a result, the properties of the obtained foamed sheet will change with time, the thermal resistance and adaptability to lamination thereof are deteriorated and sanitary troubles are caused.